This invention concerns an electrophysiological visual field measurement technique for the objective measurement of the visual field. The technique will be known as xe2x80x9cVisual Evoked Potential Objective Perimetryxe2x80x9d or xe2x80x9cVEPO-Perimetryxe2x80x9d. There is a strong demand for such a measure to supplement the variable performances seen on automated perimetry and other psychophysical tests in the evaluation of glaucoma and other disorders of vision.
The conventional full field visual evoked potential (VEP) provides information mostly about the central visual field. It is reported to be abnormal in about half of the population with glaucomaxe2x80x94a disease which is one of the commonest causes of blindness. Since many patients can have normal responses, it does not have good discriminatory power for the detection of the disease. The variable findings have previously been explained by the fact that the VEP predominantly reflects macular function and in glaucoma the damage tends to affect central vision late in the disease.
There are several studies using stimulation of parts of the visual field to record VEPs. These have employed half-fields, quadrants, segments. annulus or peripheral field vs central field, and also local stimulation using light emitting diodes. These techniques greatly improve detection of the peripheral visual field defects, compared to full central stimulation. However, significantly higher responses from stimulation of upper hemiretina (lower visual field) compared with lower hemiretina (upper visual field) has often been reported.
A major advancement in stimulus and recording technology has recently been introduced which enables the presentation of a multifocal stimulus. This is now commercially available as the VERISxe2x80x94Scientific(trademark) system (Electro-Diagnostic Imaging, Inc., San Francisco) or Retiscan (Roland Instruments Wiesbaden, Germany) . These systems provide the opportunity for topographical analysis of recordings, with the capability of examining the effects of sequential flashes. This adds a time domain to the analysis and allows examination for temporal non-linearities in the response.
Visual stimuli are presented in a preset number of hexagons in arrays (or segments in a dart board) with the possibility of flash or pattern stimuli within each area. The stimulus areas can be cortically scaled which means that the area of each dart board segment increases with eccentricity, proportional to cortical magnification.
With VERIS or Retiscan it is possible to record a detailed multifocal ERG or VEP that show a topographic distribution of signal amplitudes. While results for ERG amplitudes are useful in delineating areas of outer retinal damage in some diseases (eg retinal dystrophies), they have not been shown to correspond with areas of glaucomatous nerve fibre loss and associated visual field defects. Using a multifocal VEP response recorded and analysed with the appropriate technique however, glaucoma field defects can be detected. The additional technique required for the appropriate extraction of the multifocal pattern VEP signal is the subject of this patent.
For VEP recording, the traditional and conventional electrode placement has been monopolar (occipito-frontal), with an active electrode (Oz) placed on the back of the head 2 cm above the inion, and a reference electrode (Fz) placed on the scalp at the front of the head; with a ground electrode placed on the earlobe.
The invention is a method of measuring the electrophysiological visual field. comprising the steps of:
placing a pair of electrodes around the inion on the scalp overlying the visual cortex of the brain, in addition to a ground electrode;
visually stimulating an eye; and,
recording the data signals picked up by the electrodes. The technique is an objective method for assessing the visual field.
The data signals may be used to produce a VEP trace array.
The pair of electrodes are placed with the inion between them. They may be in line with the inion, or they may be placed in triangular relationship with the inion. The electrodes may be anywhere within a distance of 6 cm from the inion.
The electrodes may be placed at equal distances above and below the inion. and for instance may be 2 cm above and 2 cm below the inion. This placement may be termed bipolar occipital straddle or BOS placement.
An electrode may be placed at the position of the upper BOS electrode and another lower down on the midline below the position of the lower of the BOS electrodes, to measure a dipole between the upper of the BOS electrodes and the new electrode. The lower electrode may be 4 cm below the inion. This placement is called extended bipolar occipital.
An electrode may be placed on either side of the inion. The electrodes may be 4 cm on either side of the inion. This placement is called horizontal bipolar.
An electrode may be placed to the right of the inion and another below the inion. The electrode on the right hand side may be 4 cm to the right of the inion, and the lower electrode may be 4 cm below the inion. This placement is called right oblique.
An electrode may be placed to the left of the inion and another below the inion. The electrode on the left hand side may be 4 cm to the left of the inion, and the lower electrode may be 4 cm below the inion. This placement is called left oblique.
Previous attempts at field mapping have used electrode positions above the inion. Bipolar leads overlying the active occipital or striate cortex provide a superior assessment of the VEP from peripheral parts of the visual field. The projection of dipoles originating in the striate cortex subserving upper and lower hemi-fields onto the linking line between recording electrodes vertically straddling the inion is of similar magnitude but opposite polarity. This produces VEP signals from averaged upper and lower hemi-fields of similar amplitude but reverse polarity. As a result this placement produces approximately equal responses from the upper and lower hemifields.
The extended vertical BOS position with the lower electrode 4 cm below the inion improves the signal response compared to the standard BOS position (2 cm above and below). The horizontal bipolar electrodes provide a much greater signal from the test points along the horizontal meridian of the visual field. Improved detection in this area is extremely important for the application of objective perimetry to the detection of glaucoma. The oblique electrodes can also enhance the signal along the vertical midline of the visual field of the opposite side.
An electrode holder in the form of a convex cross, with a fixation strap across the forehead, is suggested to cover the electrode positions described above. It has the advantages of improving electrode contact, standardising electrode placement between tests and reducing muscle noise and artefacts.
The method may use unique electrode placement with a combined multiple channel VEP response. The post-recording analysis may involve an asymmetry analysis between eyes of the same subject, and an original scaling algorithm to reduce inter-subject variability. The method may utilise existing multifocal stimulation techniques.
Combination of Responses from Multiple Channels
One or more additional bipolar electrodes may be placed to record other channels of data input. A VEP may be recorded with more than one pair of bipolar electrodes to produce a multi-channel recording. Such a recording may then be combined in a single trace array to represent the visual field.
Ideally at least four channels are required to cover all possible dipole orientations. The greatest amplitude derived from all recorded channels at each individual point of the visual field is determined. It is then assigned to that point as the optimal signal. and its amplitude used as a measure of response of the visual pathway. The amplitude of signals within the combined array are subsequently used for data analysis.
Any multifocal stimulator (either existing equipment such as VERIS or Retiscan, or future systems) can be used to generate a cortically-scaled stimulus and extract a response. Raw data signals can be amplified by any biological amplifier by at least 100,000 times.
Multi-channel recording of the multifocal VEP using these electrode positions provides a considerable advantage in objective visual field mapping. By sampling from different bipolar electrode positions variously oriented around the striate cortex, an optimal response can be determined from each point within the field.
Asymmetry Analysis Between Eyes of an Individual Subject
The VEP trace from a particular sector of the combined trace array of one eye is compared to the amplitude of VEP from the corresponding area of the fellow eye and an amplitude ratio is calculated. The ratio is then compared to the normal ratio from the corresponding segment of the visual field from a normal data base and a probability of abnormality is calculated. This minimises the effects of within-eye asymmetry on interpretation of the response, and may help to reveal early pathological changes.
Scaling
To minimise the effects of inter-subject variability of amplitude, a scaling algorithm is employed to normalise data. This is based on the calculation of the largest responses within the field compared to normal values for that particular point. For instance using the tenth largest ratio of amplitude from both eyes to the normal mean value for the same point. The amplitudes of all points are then scaled up or down according to the ratio determined. This technique more effectively isolates visual field defects.
The combination of these recording and analysis techniques provides a form of objective perimetry, termed VEPO-Perimetry, which has potential advantages for clinical application particularly for the diagnosis and monitoring of glaucoma.